Conventionally, offshore oil rig installations comprise floating platforms connected to a wellhead and anchored to the ground by means of anchoring chains.
These platforms, of generally square horizontal cross-section, may have sides of several tens of meters, and a weight liable to reach several tens of thousands of tons (see several hundreds of thousands of tons).
They support not only a whole means required for extraction of oil, but possibly also means for the transformation thereof; sometimes also, they include equipment intended to provide for a human presence on board.
For the anchoring thereof, several groups of chains (also called ground tackles) are generally used, each of these groups being arranged at an angle of the platform.
Each anchoring group includes several chains (for example, three to eight) arranged parallel to each other.
Each anchoring chain consists in a chain of metal links, each of which has a length of a few tenth of centimeters and is made from a wire having for example a diameter of 9 to 20 cm.
The lower end of each of these anchoring chains includes means for fixation to the ground, by means of a block buried in the sea floor. The upper end thereof extends up to a control station that is arranged on the side of the platform, above the waterline thereof, for operation by a tensioner winch.
Between the upper and lower ends, an intermediate section of these chains is associated with a device commonly called “fairlead”.
These fairleads are fixed to the platform, generally under the level of the waterline.
The tension applied to each anchoring chain by the tensioner winch associated therewith is locked by a locking means, some of which may be provided within the fairlead itself, in the form, for example, of a jaw composed of two jaw members (or cheeks) articulated about axes of rotation parallel to each other.
These fairleads also provide for guiding a change of direction of the associated anchoring chain between, on the one hand, an upstream section extending vertically from the control station, and on the other hand, a downstream section extending in an inclined manner down to the block buried in the sea floor. The term “upstream” generally refers to a direction of the chain from the fairlead toward the winch, whereas “downstream” generally refers to a direction from the fairlead toward the ground.
Fairleads of this type, described for example in the documents U.S. Pat. No. 5,845,893 and WO-2013/088082, include two structures:
(i) an upstream structure for the fastening of the fairlead to the floating installation, carrying the guide means for guiding a change of direction of the anchoring chain between the upstream section and the downstream section, and
(ii) a downstream structure, including the means for locking the anchoring chain in translation.
The downstream structure is assembled with the upstream structure through pivot connection means defining a horizontal axis of rotation.
This downstream structure is hence adapted to pivot about this horizontal axis of rotation, in an admissible angular sector, so as to fit to the inclination of the downstream section of the anchoring chain.
However, in practice, with the fairleads as described in these documents U.S. Pat. No. 5,845,893 and WO-2013/088082, the intermediate section of the anchoring chain is liable to undergo undesirable tensions as a function of the angular position of the downstream structure.
Indeed, this intermediate section may be subjected to a bending that is unsuited for certain inclinations of the downstream structure with respect to the guide means carried by the upstream structure.
It is also known, from the document FR-2 601 322, a fairlead for guiding an anchoring chain comprising two structures:
(i) an upstream fastening support, integral with the floating body which is desired to be anchored, and
(ii) a bent downstream element including means for locking the chain, in the form of a ratchet, and guide means for guiding a change of direction of the chain.
The upstream fastening support and the downstream bent element are assembled by pivot connection means that define a horizontal axis of rotation.
However, these pivot connection means are offset under the line of passage of the chain, so as not to hamper the displacement thereof, which makes the structure bulky and harms the strength thereof.